This invention relates to an automatic compression relief mechanism for internal combustion engines and more particularly to centrifugally responsive apparatus by which, during the compression stroke of an internal combustion engine, compression is partially relieved at engine cranking speeds, but is allowed to build up to full value at running speeds.
Very generally the compression relief mechanism of the present invention is of the type disclosed in U.S. Pat. Nos. 3,314,408, 3,620,203, 3,901,199 and 3,981,289. In each of those prior devices, as in the mechanism of this invention, the valve that controls the flow of gas through a port communicating with the engine combustion chamber is held slightly off its seat during the compression stroke, at cranking speeds of the engine, to allow a certain amount of gas to be displaced through the port, thus relieving the force of compression that opposes cranking rotation of the engine crankshaft. At running speeds, the valve is permitted to remain seated during the compression stroke so that the normal performance of the engine is not impaired.
In the conventional compression release mechanism, the operation of the exhaust valve is controlled by an auxiliary cam member on the camshaft. The auxiliary cam member is arranged to move between an operative, radially extending position, in which the auxiliary cam member effects the desired unseating of the exhaust valve for compression relief, and a retracted inoperative position. The position of the auxiliary cam member is in turn controlled by a centrifugally actuated flyweight that is carried by the camshaft and is mounted for movement relative to the camshaft between defined limits in directions transverse to the camshaft axis. The flyweight is biased to one of its limits of motion and is moved to its other limit of motion by centrifugal force. A connection between the flyweight and the cam member translates the bias produced motion of the flyweight towards its one limit into movement of the cam member towards its extended or operative position, while movement of the flyweight under centrifugal force toward its other limit will retract the cam member to its inoperative position.
The prior compression relief expedients of the general type above described require multiple parts that have relatively complicated shapes or require special tooling and/or modification of the camshaft for installation.
A simplified compression release mechanism is shown in U.S. Pat. No. 3,314,408 in which a generally U-shaped centrifugally actuated latch is pivotally connected to the camshaft. One end of the latch defines an auxiliary cam member, while the opposite end of the latch is provided with a flyweight. The cam member is biased by a spring to the operative valve unseating position, and at running speeds the latch is pivoted to move the cam member to the inoperative position. The device of U.S. Pat. No. 3,314,408 is characterized by the fact that the valve load, when the cam member is in its operative position, is born solely by the pivot pin, thus requiring specially tempered or hardened metals for the pin to prevent failure.